1. Field of the Invention
The present invention relates to a data processing apparatus for divers for efficiently calculating the non-decompression limit, a data processing method for the same, a program for executing this method, and a recording medium for storing the program.
2. Description of the Related Art
A data processing apparatus for divers, more commonly referred to as a dive computer, has various safety functions that help to assure safe diving. One of these functions calculates the non-decompression limit, that is, the limit specifying how long a diver can dive safely without risk of decompression sickness, based on the accumulation of inert gases (particularly nitrogen) in the tissues of the diver's body. Various theories are used to compute this accumulation of inert gases in the tissues, and divers preferably dive within the non-decompression limit determined by the dive computer.
Dive computers are discussed in detail in “Dive Computers, A Consumer's Guide to History, Theory, and Performance,” by Ken Loyst, et al., Watersport Publishing Inc. (1991). Diving theory is also discussed in detail in “Decompression-Decompression Sickness,” by A. A. Buhlmann, Springer, Berlin (1984). These books note the following.
1. Different body tissues absorb (in-gas) and release (out-gas) inert gases at different rates and are grouped into “tissue compartments”, or tissue types, according to the rate of inert gas absorption and release.
2. Body tissues absorb and release inert gases at an exponential rate.
3. The saturation half-time, which is the time required for a body tissue to become half saturated, is used to express the rate of inert gas absorption and release.
4. Each tissue compartment has a particular saturation half-time and maximum inert gas partial pressure at which a safe ascent to the surface is possible, and this is referred to as the maximum tolerated (inert gas) partial pressure (the M value, M0).
5. The risk of decompression sickness occurs when a diver ascends with inert gas exceeding this maximum tolerated partial pressure (M value) still dissolved in the body tissues.
6. In general recreational diving, nitrogen is the most common inert gas.
These findings are based on experience and experimental diving, and have not been fully explained physiologically. Further, these findings were not obtained by monitoring divers while diving, and are based on mathematically modeled simulations. It is clear that more accurate simulations are important not only for preventing decompression sickness but also for improving diving safety.
The non-decompression limit is the shortest time required for a particular tissue compartment to reach the maximum tolerated inert gas partial pressure. The non-decompression limit at a given depth is calculated using an exponential function or logarithmic function based on the measured depth (or water pressure).
During a single dive of approximately one hour the dive computer measures the water depth every second and calculates the non-decompression limit from the measured water depth. This requires a massive number of calculations and high battery power consumption. Dive computers are therefore unable to use the common button batteries used in wristwatches because of the danger that the battery will wear out during the dive.
Portable dive computers therefore typically use a relatively slow 4-bit or 8-bit CPU in an effort to extend battery life, but such CPUs do not have the ability to process these functions. Constants are therefore derived for the exponential functions used in the non-decompression limit equations to simplify calculation and determine approximate values.
[Problem to Be Solved]
By using a CPU with a slow processing time, conventional dive computers are unable to quickly compute the non-decompression limit at the same rate the depth is measured, that is, every second, and there is a several second delay until the results are displayed. Depth measurements must therefore be delayed to a commensurate interval of several seconds, thus diminishing the effectiveness of the dive computer.
Furthermore, advances in diving theory have increased the number of theoretical tissue compartments that must be considered when calculating the non-decompression limit from 9 to 16. In addition, the mixture of nitrogen and oxygen in the tank is variable, and helium may also be added to the breathing mix. These factors each increase the number of calculations that must be performed by the dive computer, and exceed the processing capacity of conventionally used CPUs.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.